How quickly help arrives for you and how effective it is for you will depend on where the disaster is. At Ladbroke Grove (the Paddington crash) the fire brigade were on site within a few minutes.
However this will not always be the case. It is easy to criticise the emergency services for delays and for what you might think is tardiness on their behalf, but they cannot practise for every disaster and they must ensure nothing they do puts the survivors at more risk.
Immediately after an incident they will set up control systems to assess what is required. There may be fires to fight, there will be people trapped and people badly injured. It can be easy to criticise the emergency services, but they will have followed carefully thought out plans and will be primarily focused on those badly injured and trapped.
If you are not badly injured, try as soon as possible to let someone you know that you are fine. Part of the emergency response will have involved setting up a casualty bureau and much time can be wasted tracing people who have survived. At one point during the rescue Ladbroke Grove there were 140 people listed as missing, when in fact only 1 was.
After a disaster the emergency services will take you to a triage area for assessment for injuries. If you are injured you will be assessed for appropriate treatment.
Your details should be taken by police, you may have important information as a witness, and they will need proper statistics regarding who was involved. Ask them what plans there are for getting you back home.
The media will almost certainly be trying to interview people involved. Don't get pressurised into giving one, and if you do remember that they are after information for their paper or broadcast, they may be on a deadline, and they may not have your best interests at heart.
How you react will be entirely based on your injuries and experiences, along with your own personality makeup. You may feel guilty about surviving, if others died. You may feel elated at being alive, with your senses working at 120% - it's good to be alive!
Talk to friends and family about your experiences, if you can. Remember they will have no idea what it was like. You may feel uncomfortable talking about some aspects, just talk about what you are comfortable with.
Various people may get in touch with you, your details will have been passed to involved organisations, such as insurers.
The police will want to come and take a full statement from you. When they do, try to give them as much detail as possible, which may be distressing, as you will have to go through the entire event, covering everything that happened to you.
You may be contacted by counsellors to arrange debriefing. This is likely to be group debriefing.
The companies involved may write to you. Try not to get upset by this, even if you feel that they are culpable for what you are going through.
Local media may try to get your story, if it is known that you were involved. If you don't want to get involved, say so. If you do, remember that they may well want more contact with you in the future.
The insurers may contact you. See the section on compensation before you make any decisions. You may wish to put everything behind you, and settle everything quickly, but some effects from a disaster can take months to appear. Don't do anything that you may regret later.
You should also consider appointing a lawyer to make a claim on your behalf. Normally you don't have to pay for their services, the fee is paid by the insurer.
Only you know what you have been through. Your friends, family, colleagues, employers and anyone else close to you will not know what you saw, felt, heard. You may not want to talk about it. Try to explain to them how you are feeling, even if you can't give them details.
After the initial few days and weeks, life will get back to some sort of normality. However what you will find is that as the media interest wanes, your story becomes less important to people around, and you are expected just to get on with things. This can be very hard on you, as it is such an important part of you and your life.
You may develop longer term health issues. Post Traumatic Stress Disorder (PTSD) doesn't develop immediately and you cannot be accurately assessed for it until about 6 months have passed.
Until you have been assessed by a psychiatrist and treated:
Don't move house!
Don't get divorced!
Don't change jobs!
If you make a major decision like that before you know you haven't been affected, you may well be doing it for entirely the wrong reasons.